I think it is the scrolling store from auctiva that is causing your page to be off-centered. I have the same problem and have been trying to fix it. but as yet, I haven't been able to.
I now just keep the scrolling store out of my auctions.
When you have the store enabled in auctiva it automatically puts the store at the top of your ebay listing, causing the margins on ebays page to push out which then makes the whole page off-centered slightly.
You can enter the "Store Tag" manually into your custom templates (when you create your template) but if you do that make sure to switch off the "enable store" tag under the STORE TAB ~ STORE WINDOW.
Try placing the "store tag" outside of your custom templates tables, at the bottom of your template page. It stops the scrolling to the left somewhat, but not alltogether.
I am going to try the "fixed store" instead of the scrolling store and see if that stops the off centering.